Photonic crystal fibres have in recent years proved a very valuable technology. There is, however, a need for photonic crystal fibres that may provide relatively large core sizes and relatively strong birefringence. Present photonic crystal fibres may provide strong birefringence, but have relatively small core sizes (see e.g. Hansen et al., IEEE Phot. Tech. Lett., 13, 6, 2001; Ortigosa-Blanch Opt. Lett., 25, 18, 2000). The small core size makes the fibres difficult to work with (for example difficult to couple light to them). The small core further limits the power handling capacities of the fibres. The present birefringent fibres have furthermore strongly wavelength dependent birefringence that may limit the use of the fibres.
WO-00/49436 describes a photonic crystal fibre comprising a bulk material having an arrangement of longitudinal holes and a guiding core, wherein the fibre has at-most-two-fold rotational symmetry about a longitudinal axis and as a result of that lack of symmetry, the fibre is birefringent. It is stated that the methods used to produce birefringence in standard fibres, and thus to produce standard polarisation-preserving fibres, are, in general, not directly suitable for use in photonic-crystal fibre. The same conclusion is drawn in the article by Ortigosa-Blanch et al. “Highly birefringent photonic crystal fibres”, Optics Letters, Vol. 25, No. 18, 15 Sep. 2000, pp. 1325-1327, where it is stated that a holey cladding is unable to transmit uni-axial stress to the core due to its strong mechanical anisotropy.
WO-03/100488 describes an optical fibre exhibiting special polarization properties, the fibre comprising a core region and a cladding region; the core region comprising a substantially 1D periodic structure having a substantially elliptical shape, and the cladding region comprising a number of cladding elements (typically air holes) that are placed in a cladding background material, the cladding further comprising two stress-inducing cladding element, the cladding having a background material (typically silica) and an outer, solid overcladding part. Due to the micro-structured core region and its special form, such a fibre is relatively complicated.